Re: [patch] document flash/RAID dangers

From: Ric Wheeler
Date: Tue Aug 25 2009 - 19:49:18 EST

There are storage devices that high highly undesirable properties
when they are disconnected or suffer power failures while writes are
in progress; such devices include flash devices and MD RAID 4/5/6
arrays. These devices have the property of potentially
corrupting blocks being written at the time of the power failure, and
worse yet, amplifying the region where blocks are corrupted such that
additional sectors are also damaged during the power failure.

I would strike the entire mention of MD devices since it is your assertion, not a proven fact. You will cause more data loss from common events (single sector errors, complete drive failure) by steering people away from more reliable storage configurations because of a really rare edge case (power failure during split write to two raid members while doing a RAID rebuild).

Users who use such storage devices are well advised take
countermeasures, such as the use of Uninterruptible Power Supplies,
and making sure the flash device is not hot-unplugged while the device
is being used. Regular backups when using these devices is also a
Very Good Idea.

All users who care about data integrity - including those who do not use MD5 but just regular single S-ATA disks - will get better reliability from a UPS.

Otherwise, file systems placed on these devices can suffer silent data
and file system corruption. An forced use of fsck may detect metadata
corruption resulting in file system corruption, but will not suffice
to detect data corruption.

This is very misleading. All storage "can" have silent data loss, you are making a statement without specifics about frequency.

FSCK can repair the file system metadata, but will not detect any data loss or corruption in the data blocks allocated to user files. To detect data loss properly, you need to checksum (or digitally sign) all objects stored in a file system and verify them on a regular basis.

Also helps to keep a separate list of those objects on another device so that when the metadata does take a hit, you can enumerate your objects and verify that you have not lost anything.



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